I think adding the option to choose your sim's pronoun can be done with a few UI changes and some changes to the part of the engine that selects which text to display based on your sim's pronoun (instead of basing it on gender, it's based on the selected pronoun.) I suppose they can also hide that part of the UI for users who play TS4 in a language that's already gender-neutral to begin with.
Adding custom pronouns (such as they/them) might be more difficult depending on how the text is stored. If each text just uses a template and the game inserts the appropriate pronouns based on your sim's gender, it might be easier. If each text has two versions depending on your sim's gender, they would have to go and rewrite everything to add a third version that uses they/them pronouns, which would be more time-consuming. I haven't really looked into the game's text files so I'm not sure how it works.
